Enabling chain scission of branched photoresist

ABSTRACT

By using a branched long chained chain scission polymer as a photoresist for EUV and 157 nanometer applications, a relatively higher molecular weight polymer with good mechanical properties may be achieved. In addition, by using chain scission technology, line edge roughness and resolution may be improved at the same time.

BACKGROUND

This invention relates generally to the fabrication of integratedcircuits and, particularly, to photoresist utilized in the manufacturingof integrated circuits.

A thin photoresist film is patterned and used as a sacrificial layer totransfer the pattern to the underlying substrate during themanufacturing of integrated circuits. Patterns are created in aphotoresist film as a result of exposure to radiation through a mask.The radiation causes a chemical reaction that changes the solubility ofthe photoresist. As a result, upon subsequent exposure to a developersolution, some of the photoresist may remain and other of thephotoresist may be removed resulting in the transfer of a pattern fromthe exposure mask to the semiconductor structure.

This pattern, now resident in the photoresist structure, may in turn beutilized as a mask to remove portions of the underlying semiconductorstructure using chemical etching processes. Thus, the use of photoresistenables the repeated fabrication of a large number of integratedcircuits using a single mask that repeatedly transfers the same patternto the semiconductor structure.

It has been the goal for many years in semiconductor fabrication toreduce the size of the features that are fabricated. Smaller integratedcircuits generally mean faster integrated circuits and lower costintegrated circuits. One way to reduce the feature size is to useimproved lithography. Generally, lithography improvements have been theresult of changing the wavelength of the radiation used to expose thepattern. With wavelengths that are able to define even smaller patterns,the photoresist technology will also need to be improved to realize adecreased feature size. Namely, when smaller features may be exposedthrough improvements in lithography, it is also necessary that thephotoresist be able to define the smaller features enabled by improvedlithography.

The size of the features that undergo dissolution in the photoresistultimately limits the line edge roughness (LER) and resolution that isavailable with any lithographic system. Line edge roughness is theroughness of the edge of the photoresist. This roughness is transferredto the underlying semiconductor structure in the subsequent processingsteps, adversely affecting device performance. Thus, many manufacturersof integrated circuits have focused on reducing the molecular weight ofthe polymers used in photoresist in order to reduce line edge roughnessand increase resolution.

Unfortunately, a reduction of molecular weight of these photoresistforming polymers may have a negative impact on the mechanical propertiesof the photoresist. For example, lower molecular weight photoresists maybe more likely to exhibit photoresist pattern collapse. Obviously, whenthe pattern collapses, the photoresist may be non-functional.

Thus, there is a need for better photoresists that exhibit desirableline edge roughness and improved resolution, without resist photoresistpattern collapse at relatively high resolution radiation wavelengths,such as extreme ultraviolet (EUV) or 157 nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic depiction of one embodiment of the presentinvention;

FIGS. 2A and 2B are depictions of the chemical structure of embodimentsof the present invention; and

FIGS. 3A and 3B are depictions of the chemical systems of anotherembodiment of the present invention.

DETAILED DESCRIPTION

Traditionally the difference in the solubility of the exposed andunexposed photoresist in a developer solution occurs when protectinggroups are cleaved in the exposed region. Once these groups are cleaved,the solubility of the polymer changes, usually with the cleavage ofnon-polar side groups from the polymer backbone leaving polar groups intheir place that are more soluble in an aqueous base. The entire polymermolecule may then be selectively dissolved in a developer.

The minimum value possible for resolution and line edge roughness isdetermined by the size of the polymer molecules. Furthermore, polymermolecules are known in some cases to entangle with each other, formingaggregates composed of many polymer molecules that dissolve as a singleunit, further limiting resolution and line edge roughness.

Chain scission is a chemical reaction resulting in the breaking ofchemical bonds. A chemical bond is a bond between two atoms of asequence of atoms in the constitutional units of a macromolecule, anoligomer molecule, a block, or a chain which defines the essentialgeometric representation of a polymer.

In polymers that undergo chain scission, the difference in solubilitybetween the exposed and unexposed regions is driven primarily bydifferences in molecular weight. Because the size of the polymersegments that are dissolving are small, the resolution and line edgeroughness are not limited by the size of the chain scission polymer.Since segments cleaved from polymer after chain scission undergodissolution rather than the entire polymer, the molecular weight isdecoupled from the resolution and line edge roughness.

Therefore, a relatively higher molecular weight polymer may be utilizedto improve mechanical properties such as Young's modulus and yieldstress. Also, by using branched polymers, better mechanical propertiesmay be achieved in some embodiments. In some embodiments, the molecularweight of the polymer may be greater than 10000 Daltons. The branchesmay be long chains having sizes greater than 5000 Daltons in someembodiments.

The size of the scissioned polymer segments after exposure may beengineered by changing the number of scissionable linkages in thepolymer's backbone. For example, referring to FIG. 1, a chain scissionpolymer 10 may include a polymer backbone 12 with branches 14 extendingtherefrom. Chain scission points 16 may be defined along the length ofthe branches 14 which chain scission points may be subject to a chainscission reaction.

Due to the large molecular weights that can be used with chain scissionpolymers, the mechanical properties of branched chain scissionphotoresist may be improved. The size of the polymer molecule isdecoupled from its imaging properties. Long branches may be incorporatedinto the polymer. These modifications of branch length and molecularweight improve the mechanical properties of the photoresist and reducethe severity of any photoresist collapse.

An extreme ultraviolet (EUV) photoresist may use a branched chainscission polymer such as poly hydroxystyrene-type polymer shown in FIG.2A.

The structure shown in FIG. 2A is oligo-4-hydroxystyrene withtertiarycarbonated linked branches having a stable backbone withcleavable branches. A scissionable linkage may be present between thelong chain branch and the main polymer branch so that upon irradiation,the long chain branch is cleaved from the polymer backbone.

In FIG. 2B, a branched chain scissionable polymer is illustrated whereeach repeat unit within the polymer branches and as well within thepolymer backbone can undergo scission. The structure shown in FIG. 2B isan oligo-1,4-dihydroxyphenylcarbonate-bis tertiary alcohol with appendedtertiary alcohol carbonate side chains that provide both a cleavablebackbone and cleavable branches.

In FIG. 3A, the synthesis for a scissionable branched nobornyl starpolymer is shown. In this structure, each repeat unit within the polymercan undergo scission. Compound I is norbornane dicarboxylic acid, whichis reacted with SOCl₂ to form Compound II. Compound II is a norbornanediester. Compound II is reacted with a short chain diacid, as indicatedin FIG. 3A to form the complex norbornane star, Compound III. Units ofCompound III may then be linked together to form Compound IV. TheCompound IV may be reacted with trifluoroaceticanhydride (TFFA),glycerol and NH₄OH to form Compound V, which is a scissionable branchnorbornyl star, shown in FIG. 3B.

The examples shown in the FIGS. 2 and 3 illustrate different numbers ofscissionable linkages and branching configurations that are possiblewithin a chain scission polymer. The scissionable linkages may also bedistributed at strategic intervals within the polymer. As well, theratio of branched to straight chain polymer may be varied to meetspecific lithographic requirements.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

1-9. (canceled)
 10. A photoresist precursor for forming a photoresistcomprising: a branched chain scission polymer.
 11. The precursor ofclaim 10 wherein said branched chain scission polymer includesscissionable and non-scissionable linkages.
 12. The precursor of claim10 including a scionable linkage in a branch of said polymer.
 13. Theprecursor of claim 10 including a polymer having a molecular weightgreater than 10,000 Daltons.
 14. The precursor of claim 10 including apolymer having a branch having a molecular weight greater than 5000Daltons
 15. The precursor of claim 10 wherein said polymer includesoligo-4-hydroxystyrene.
 16. The precursor of claim 15 including atertiary carbonated link branch.
 17. The precursor of claim 15 includingan oligo-1,4-dihydroxy-phenylcarbonate-bis tertiary alcohol.
 18. Theprecursor of claim 17 including an appended tertiary alcohol carbonateside chain on said polymer.